Method of addressing messages and communications systems

ABSTRACT

A method of and apparatus for establishing wireless communications between an interrogator and individual ones of multiple wireless identification devices, the method comprising utilizing a tree search method to establish communications without collision between the interrogator and individual ones of the multiple wireless identification devices, a search tree being defined for the tree search method, the tree having multiple levels respectively representing subgroups of the multiple wireless identification devices, the method further comprising starting the tree search at a selectable level of the search tree. A communications system comprising an interrogator, and a plurality of wireless identification devices configured to communicate with the interrogator in a wireless fashion, the respective wireless identification devices having a unique identification number, the interrogator being configured to employ a tree search technique to determine the unique identification numbers of the different wireless identification devices so as to be able to establish communications between the interrogator and individual ones of the multiple wireless identification devices without collision by multiple wireless identification devices attempting to respond to the interrogator at the same time, wherein the interrogator is configured to start the tree search at a selectable level of the search tree. In one embodiment, the interrogator transmits a first request indicating a subgroup of random numbers out of a total number of possible random numbers. The wireless identification devices each determine if the random number generated by each wireless identification device falls within the subgroup, and if so, the wireless identification device responds to the interrogator. If a collision between wireless identification device responses is detected by the interrogator, the interrogator transmits a second request indicating a subgroup of random numbers.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. patent application Ser. No. 09/026,043,filed Feb. 19, 1998, and titled “Method of Addressing Messages andCommunications System” now U.S. Pat. No. 6,118,789. More than onereissue application has been filed for the reissue of U.S. Pat. No.6,307,847, which reissue applications are the initial reissueapplication Ser. No. 10/693,696, filed Oct. 23, 2003, now Re. 41,530, acontinuation reissue application Ser. No. 11/859,360, filed Sep. 21,2007, a continuation reissue application Ser. No. 11/859,364, filed Sep.21, 2007, a continuation reissue application Ser. No. 12/493,542, filedJun. 29, 2009, and the present continuation reissue application which isa continuation application of U.S. patent application Ser. No.10/693,696, filed Oct. 23, 2003, now Re. 41,530, which is a reissueapplication of U.S. Pat. No. 6,307,847 filed Jul. 12, 2000 and titled“Method of Addressing Messages and Communications Systems”, which is acontinuation application of U.S. patent application Ser. No. 09/026,043,filed Feb. 19, 1998, and titled “Method of Addressing Messages andCommunications System”, now U.S. Pat. No. 6,118,789, each of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to communications protocols and to digital datacommunications. Still more particularly, the invention relates to datacommunications protocols in mediums such as radio communication or thelike. The invention also relates to radio frequency identificationdevices for inventory control, object monitoring, determining theexistence, location or movement of objects, or for remote automatedpayment.

BACKGROUND OF THE INVENTION

Communications protocols are used in various applications. For example,communications protocols can be used in electronic identificationsystems. As large numbers of objects are moved in inventory, productmanufacturing, and merchandising operations, there is a continuouschallenge to accurately monitor the location and flow of objects.Additionally, there is a continuing goal to interrogate the location ofobjects in an inexpensive and streamlined manner. One way of trackingobjects is with an electronic identification system.

One presently available electronic identification system utilizes amagnetic coupling system. In some cases, an identification device may beprovided with a unique identification code in order to distinguishbetween a number of different devices. Typically, the devices areentirely passive (have no power supply), which results in a small andportable package. However, such identification systems are only capableof operation over a relatively short range, limited by the size of amagnetic field used to supply power to the devices and to communicatewith the devices.

Another wireless electronic identification system utilizes a largeactive transponder device affixed to an object to be monitored whichreceives a signal from an interrogator. The device receives the signal,then generates and transmits a responsive signal. The interrogationsignal and the responsive signal are typically radio-frequency (RF)signals produced by an RF transmitter circuit. Because active deviceshave their own power sources, and do not need to be in close proximityto an interrogator or reader to receive power via magnetic coupling.Therefore, active transponder devices tend to be more suitable forapplications requiring tracking of a tagged device that may not be inclose proximity to an interrogator. For example, active transponderdevices tend to be more suitable for inventory control or tracking.

Electronic identification systems can also be used for remote payment.For example, when a radio frequency identification device passes aninterrogator at a toll booth, the toll booth can determine the identityof the radio frequency identification device, and thus of the owner ofthe device, and debit an account held by the owner for payment of tollor can receive a credit card number against which the toll can becharged. Similarly, remote payment is possible for a variety of othergoods or services.

A communication system typically includes two transponders: a commanderstation or interrogator, and a responder station or transponder devicewhich replies to the interrogator.

If the interrogator has prior knowledge of the identification number ofa device which the interrogator is looking for, it can specify that aresponse is requested only from the device with that identificationnumber. Sometimes, such information is not available. For example, thereare occasions where the interrogator is attempting to determine which ofmultiple devices are within communication range.

When the interrogator sends a message to a transponder device requestinga reply, there is a possibility that multiple transponder devices willattempt to respond simultaneously, causing a collision, and thus causingan erroneous message to be received by the interrogator. For example, ifthe interrogator sends out a command requesting that all devices withina communications range identify themselves, and gets a large number ofsimultaneous replies, the interrogator may not be able to interpret anyof these replies. Thus, arbitration schemes are employed to permitcommunications free of collisions.

In one arbitration scheme or system, described in commonly assigned U.S.Pat. Nos. 5,627,544; 5,583,850; 5,500,650; and 5,365,551, all toSnodgrass et al. and all incorporated herein by reference, theinterrogator sends a command causing each device of a potentially largenumber of responding devices to select a random number from a knownrange and use it as that device's arbitration number. By transmittingrequests for identification to various subsets of the full range ofarbitration numbers, and checking for an error-free response, theinterrogator determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator is able to conduct subsequent uninterrupted communicationwith devices, one at a time, by addressing only one device.

Another arbitration scheme is referred to as the Aloha or slotted Alohascheme. This scheme is discussed in various references relating tocommunications, such as Digital Communications: Fundamentals andApplications, Bernard Sklar, published January 1988 by Prentice Hall. Inthis type of scheme, a device will respond to an interrogator using oneof many time domain slots selected randomly by the device. A problemwith the Aloha scheme is that if there are many devices, or potentiallymany devices in the field (i.e. in communications range, capable ofresponding) then there must be many available slots or many collisionswill occur. Having many available slots slows down replies. If themagnitude of the number of devices in a field is unknown, then manyslots are needed. This results in the system slowing down significantlybecause the reply time equals the number of slots multiplied by the timeperiod required for one reply.

An electronic identification system which can be used as a radiofrequency identification device, arbitration schemes, and variousapplications for such devices are described in detail in commonlyassigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29,1996, and now U.S. Pat. No. 6,130,602, which is incorporated herein byreference.

SUMMARY OF THE INVENTION

The invention provides a wireless identification device configured toprovide a signal to identify the device in response to an interrogationsignal.

One aspect of the invention provides a method of establishing wirelesscommunications between an interrogator and individual ones of multiplewireless identification devices. The method comprises utilizing a treesearch method to establish communications without collision between theinterrogator and individual ones of the multiple wireless identificationdevices. A search tree is defined for the tree search method. The treehas multiple levels respectively representing subgroups of the multiplewireless identification devices. The method further comprising startingthe tree search at a selectable level of the search tree. In one aspectof the invention, the method further comprises determining the maximumpossible number of wireless identification devices that couldcommunicate with the interrogator, and selecting a level of the searchtree based on the determined maximum possible number of wirelessidentification devices that could communicate with the interrogator. Inanother aspect of the invention, the method further comprises startingthe tree search at a level determined by taking the base two logarithmof the determined maximum possible number, wherein the level of the treecontaining all subgroups is considered level zero, and lower levels arenumbered consecutively.

Another aspect of the invention provides a communications systemcomprising an interrogator, and a plurality of wireless identificationdevices configured to communicate with the interrogator in a wirelessfashion. The respective wireless identification devices have a uniqueidentification number. The interrogator is configured to employ a treesearch technique to determine the unique identification numbers of thedifferent wireless identification devices so as to be able to establishcommunications between the interrogator and individual ones of themultiple wireless identification devices without collision by multiplewireless identification devices attempting to respond to theinterrogator at the same time. The interrogator is configured to startthe tree search at a selectable level of the search tree.

One aspect of the invention provides a radio frequency identificationdevice comprising an integrated circuit including a receiver, atransmitter, and a microprocessor. In one embodiment, the integratedcircuit is a monolithic single die single metal layer integrated circuitincluding the receiver, the transmitter, and the microprocessor. Thedevice of this embodiment includes an active transponder, instead of atransponder which relies on magnetic coupling for power, and thereforehas a much greater range.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a high level circuit schematic showing an interrogator and aradio frequency identification device embodying the invention.

FIG. 2 is a front view of a housing, in the form of a badge or card,supporting the circuit of FIG. 1 according to one embodiment theinvention.

FIG. 3 is a front view of a housing supporting the circuit of FIG. 1according to another embodiment of the invention.

FIG. 4 is a diagram illustrating a tree splitting sort method forestablishing communication with a radio frequency identification devicein a field of a plurality of such devices.

FIG. 5. is a diagram illustrating a modified tree splitting sort methodfor establishing communication with a radio frequency identificationdevice in a field of a plurality of such devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIG. 1 illustrates a wireless identification device 12 in accordancewith one embodiment of the invention. In the illustrated embodiment, thewireless identification device is a radio frequency data communicationdevice 12, and includes RFID circuitry 16. The device 12 furtherincludes at least one antenna 14 connected to the circuitry 16 forwireless or radio frequency transmission and reception by the circuitry16. In the illustrated embodiment, the RFID circuitry is defined by anintegrated circuit as described in the above-incorporated patentapplication Ser. No. 08/705,043, filed Aug. 29, 1996, now U.S. Pat. No.6,130,602. Other embodiments are possible. A power source or supply 18is connected to the integrated circuit 16 to supply power to theintegrated circuit 16. In one embodiment, the power source 18 comprisesa battery.

The device 12 transmits and receives radio frequency communications toand from an interrogator 26. An exemplary interrogator is described incommonly assigned U.S. patent application Ser. No. 08/907,689, filedAug. 8, 1997 and, now U.S. Pat. No. 6,289,209, which is incorporatedherein by reference. Preferably, the interrogator 26 includes an antenna28, as well as dedicated transmitting and receiving circuitry, similarto that implemented on the integrated circuit 16.

Generally, the interrogator 26 transmits an interrogation signal orcommand 27 via the antenna 28. The device 12 receives the incominginterrogation signal via its antenna 14. Upon receiving the signal 27,the device 12 responds by generating and transmitting a responsivesignal or reply 29. The responsive signal 29 typically includesinformation that uniquely identifies, or labels the particular device 12that is transmitting, so as to identify any object or person with whichthe device 12 is associated.

Although only one device 12 is shown in FIG. 1, typically there will bemultiple devices 12 that correspond with the interrogator 26, and theparticular devices 12 that are in communication with the interrogator 26will typically change over time. In the illustrated embodiment in FIG.1, there is no communication between multiple devices 12. Instead, thedevices 12 respectively communicate with the interrogator 26. Multipledevices 12 can be used in the same field of an interrogator 26 (i.e.,within communications range of an interrogator 26).

The radio frequency data communication device 12 can be included in anyappropriate housing or packaging. Various methods of manufacturinghousings are described in commonly assigned U.S. patent application Ser.No. 08/800,037, filed Feb. 13, 1997, and now U.S. Pat. No. 5,988,510,which is incorporated herein by reference.

FIG. 2 shows but one embodiment in the form of a card or badge 19including a housing 11 of plastic or other suitable material supportingthe device 12 and the power supply 18. In one embodiment, the front faceof the badge has visual identification features such as graphics, text,information found on identification or credit cards, etc.

FIG. 3 illustrates but one alternative housing supporting the device 12.More particularly, FIG. 3 shows a miniature housing 20 encasing thedevice 12 and power supply 18 to define a tag which can be supported byan object (e.g., hung from an object, affixed to an object, etc.).Although two particular types of housings have been disclosed, thedevice 12 can be included in any appropriate housing.

If the power supply 18 is a battery, the battery can take any suitableform. Preferably, the battery type will be selected depending on weight,size, and life requirements for a particular application. In oneembodiment, the battery 18 is a thin profile button-type cell forming asmall, thin energy cell more commonly utilized in watches and smallelectronic devices requiring a thin profile. A conventional button-typecell has a pair of electrodes, an anode formed by one face and a cathodeformed by an opposite face. In an alternative embodiment, the powersource 18 comprises a series connected pair of button type cells.Instead of using a battery, any suitable power source can be employed.

The circuitry 16 further includes a backscatter transmitter and isconfigured to provide a responsive signal to the interrogator 26 byradio frequency. More particularly, the circuitry 16 includes atransmitter, a receiver, and memory such as is described in U.S. patentapplication Ser. No. 08/705,043, now U.S. Pat. No. 6,130,602.

Radio frequency identification has emerged as a viable and affordablealternative to tagging or labeling small to large quantities of items.The interrogator 26 communicates with the devices 12 via anelectromagnetic link, such as via an RF link (e.g., at microwavefrequencies, in one embodiment), so all transmissions by theinterrogator 26 are heard simultaneously by all devices 12 within range.

If the interrogator 26 sends out a command requesting that all devices12 within range identify themselves, and gets a large number ofsimultaneous replies, the interrogator 26 may not be able to interpretany of these replies. Therefore, arbitration schemes are provided.

If the interrogator 26 has prior knowledge of the identification numberof a device 12 which the interrogator 26 is looking for, it can specifythat a response is requested only from the device 12 with thatidentification number. To target a command at a specific device 12,(i.e., to initiate point-on-point communication), the interrogator 26must send a number identifying a specific device 12 along with thecommand. At start-up, or in a new or changing environment, theseidentification numbers are not known by the interrogator 26. Therefore,the interrogator 26 must identify all devices 12 in the field (withincommunication range) such as by determining the identification numbersof the devices 12 in the field. After this is accomplished,point-to-point communication can proceed as desired by the interrogator26.

Generally speaking, RFID systems are a type of multi-accesscommunication system. The distance between the interrogator 26 anddevices 12 within the field is typically fairly short (e.g., severalmeters), so packet transmission time is determined primarily by packetsize and baud rate. Propagation delays are negligible. In such systems,there is a potential for a large number of transmitting devices 12 andthere is a need for the interrogator 26 to work in a changingenvironment, where different devices 12 are swapped in and outfrequently (e.g., as inventory is added or removed). In such systems,the inventors have determined that the use of random access methods workeffectively for contention resolution (i.e., for dealing with collisionsbetween devices 12 attempting to respond to the interrogator 26 at thesame time).

RFID systems have some characteristics that are different from othercommunications systems. For example, one characteristic of theillustrated RFID systems is that the devices 12 never communicatewithout being prompted by the interrogator 26. This is in contrast totypical multiaccess systems where the transmitting units operate moreindependently. In addition, contention for the communication medium isshort lived as compared to the ongoing nature of the problem in othermultiaccess systems. For example, in a RFID system, after the devices 12have been identified, the interrogator can communicate with them in apoint-to-point fashion. Thus, arbitration in a RFID system is atransient rather than steady-state phenomenon. Further, the capabilityof a device 12 is limited by practical restrictions on size, power, andcost. The lifetime of a device 12 can often be measured in terms ofnumber of transmissions before battery power is lost. Therefore, one ofthe most important measures of system performance in RFID arbitration istotal time required to arbitrate a set of devices 12. Another measure ispower consumed by the devices 12 during the process. This is in contrastto the measures of throughput and packet delay in other types ofmultiaccess systems.

FIG. 4 illustrates one arbitration scheme that can be employed forcommunication between the interrogator and devices 12. Generally, theinterrogator 26 sends a command causing each device 12 of a potentiallylarge number of responding devices 12 to select a random number from aknown range and use it as that device's arbitration number. Bytransmitting requests for identification to various subsets of the fullrange of arbitration numbers, and checking for an error-free response,the interrogator 26 determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator 26 is able to conduct subsequent uninterruptedcommunication with devices 12, one at a time, by addressing only onedevice 12.

Three variables are used: an arbitration value (AVALUE), an arbitrationmask (AMASK), and a random value ID (RV). The interrogator sends anIdentify command (IdentifyCmnd) causing each device of a potentiallylarge number of responding devices to select a random number from aknown range and use it as that device's arbitration number. Theinterrogator sends an arbitration value (AVALUE) and an arbitration mask(AMASK) to a set of devices 12. The receiving devices 12 evaluate thefollowing equation: (AMASK & AVALUE)==(AMASK & RV) wherein “&” is abitwise AND function, and wherein “==” is an equality function. If theequation evaluates to “1” (TRUE), then the device 12 will reply. If theequation evaluates to “0” (FALSE), then the device 12 will not reply. Byperforming this in a structured manner, with the number of bits in thearbitration mask being increased by one each time, eventually a device12 will respond with no collisions. Thus, a binary search treemethodology is employed.

An example using actual numbers will now be provided using only fourbits, for simplicity, reference being made to FIG. 4. In one embodiment,sixteen bits are used for AVALUE and AMASK. Other numbers of bits canalso be employed depending, for example, on the number of devices 12expected to be encountered in a particular application, on desired costpoints, etc.

Assume, for this example, that there are two devices 12 in the field,one with a random value (RV) of 1100 (binary), and another with a randomvalue (RV) of 1010 (binary). The interrogator is trying to establishcommunications without collisions being caused by the two devices 12attempting to communicate at the same time.

The interrogator sets AVALUE to 0000 (or “don't care” for all bits, asindicated by the character “X” in FIG. 4) and AMASK to 0000. Theinterrogator transmits a command to all devices 12 requesting that theyidentify themselves. Each of the devices 12 evaluate (AMASK &AVALUE)==(AMASK & RV) using the random value RV that the respectivedevices 12 selected. If the equation evaluates to “1” (TRUE), then thedevice 12 will reply. If the equation evaluates to “0” (FALSE), then thedevice 12 will not reply. In the first level of the illustrated tree,AMASK is 0000 and anything bitwise ANDed with all zeros results in allzeros, so both the devices 12 in the field respond, and there is acollision.

Next, the interrogator sets AMASK to 0001 and AVALUE to 0000 andtransmits an identify command. Both devices 12 in the field have a zerofor their least significant bit, and (AMASK & AVALUE)==(AMASK & RV) willbe true for both devices 12. For the device 12 with a random value of1100, the left side of the equation is evaluated as follows (0001 &0000)=0000. The right side is evaluated as (0001 & 1100)=0000. The leftside equals the right side, so the equation is true for the device 12with the random value of 1100. For the device 12 with a random value of1010, the left side of the equation is evaluated as (0001 & 0000)=0000.The right side is evaluated as (0001 & 1010)=0000. The left side equalsthe right side, so the equation is true for the device 12 with therandom value of 1010. Because the equation is true for both devices 12in the field, both devices 12 in the field respond, and there is anothercollision.

Recursively, the interrogator next sets AMASK to 0011 with AVALUE stillat 0000 and transmits an Identify command. (AMASK & AVALUE)==(AMASK &RV) is evaluated for both devices 12. For the device 12 with a randomvalue of 1100, the left side of the equation is evaluated as follows(0011 & 0000)=0000. The right side is evaluated as (0011 & 1100)=0000.The left side equals the right side, so the equation is true for thedevice 12 with the random value of 1100, so this device 12 responds. Forthe device 12 with a random value of 1010, the left side of the equationis evaluated as (0011 & 0000)=0000. The right side is evaluated as (0011& 1010)=0010. The left side does not equal the right side, so theequation is false for the device 12 with the random value of 1010, andthis device 12 does not respond. Therefore, there is no collision, andthe interrogator can determine the identity (e.g., an identificationnumber) for the device 12 that does respond.

De-recursion takes place, and the devices 12 to the right for the sameAMASK level are accessed when AVALUE is set at 0010, and AMASK is set to0011.

The device 12 with the random value of 1010 receives a command andevaluates the equation (AMASK & AVALUE)==(AMASK & RV). The left side ofthe equation is evaluated as (0011 & 0010)=0010. The right side of theequation is evaluated as (0011 & 1010)=0010. The right side equals theleft side, so the equation is true for the device 12 with the randomvalue of 1010. Because there are no other devices 12 in the subtree, agood reply is returned by the device 12 with the random value of 1010.There is no collision, and the interrogator 26 can determine theidentity (e.g., an identification number) for the device 12 that doesrespond.

By recursion, what is meant is that a function makes a call to itself.In other words, the function calls itself within the body of thefunction. After the called function returns, de-recursion takes placeand execution continues at the place just after the function call; i.e.at the beginning of the statement after the function call.

For instance, consider a function that has four statements (numbered1,2,3,4 ) in it, and the second statement is a recursive call. Assumethat the fourth statement is a return statement. The first time throughthe loop (iteration 1) the function executes the statement 2 and(because it is a recursive call) calls itself causing iteration 2 tooccur. When iteration 2 gets to statement 2, it calls itself makingiteration 3. During execution in iteration 3 of statement 1, assume thatthe function does a return. The information that was saved on the stackfrom iteration 2 is loaded and the function resumes execution atstatement 3 (in iteration 2), followed by the execution of statement 4which is also a return statement. Since there are no more statements inthe function, the function de-recurses to iteration 1. Iteration 1, hadpreviously recursively called itself in statement 2. Therefore, it nowexecutes statement 3 (in iteration 1 ). Following that it executes areturn at statement 4. Recursion is known in the art.

Consider the following code which can be used to implement operation ofthe method shown in FIG. 4 and described above.

Arbitrate(AMASK, AVALUE)  {  collision=IdentifyCmnd(AMASK, AVALUE)  if(collision) then     {       /* recursive call for left side */     Arbitrate((AMASK>>l)+l, AVALUE)       /* recursive call for rightside */    Arbitrate((AMASK>>l)+l, AVALUE+(AMASK+1))   } /* endif */ }/*return */

The symbol “<<” represents a bitwise left shift. “<<” means shift leftby one place. Thus, 0001<<1 would be 0010. Note, however, that AMASK isoriginally called with a value of zero, and 0000<<1 is still 0000.Therefore, for the first recursive call, AMASK=(AMASK<<1)+1. So for thefirst recursive call, the value of AMASK is 0000+0001=0001. For thesecond call, AMASK=(0001<<)+1=0010+1=0011. For the third recursive call,AMASK=(0011<<1)+1=0110+1=0111.

The routine generates values for AMASK and AVALUE to be used by theinterrogator in an identify command “IdentifyCmnd.” Note that theroutine calls itself if there is a collision. De-recursion occurs whenthere is no collision. AVALUE and AMASK would have values such as thefollowing assuming collisions take place all the way down to the bottomof the tree.

AVALUE AMASK 0000 0000 0000 0001 0000 0011 0000 0111 0000  1111* 1000 1111* 0100 0111 0100  1111* 1100  1111*

This sequence of AMASK, AVALUE binary numbers assumes that there arecollisions all the way down to the bottom of the tree, at which pointthe Identify command sent by the interrogator is finally successful sothat no collision occurs. Rows in the table for which the interrogatoris successful in receiving a reply without collision are marked with thesymbol “*”. Note that if the Identify command was successful at, forexample, the third line in the table then the interrogator would stopgoing down that branch of the tree and start down another, so thesequence would be as shown in the following table.

AVALUE AMASK 0000 0000 0000 0001 0000  0011* 0010 0011 . . . . . .

This method is referred to as a splitting method. It works by splittinggroups of colliding devices 12 into subsets that are resolved in turn.The splitting method can also be viewed as a type of tree search. Eachsplit moves the method one level deeper in the tree.

Either depth-first or breadth-first traversals of the tree can beemployed Depth first traversals are performed by using recursion, as isemployed in the code listed above. Breadth-first traversals areaccomplished by using a queue instead of recursion. The following is anexample of code for performing a breadth-first traversal.

Arbitrate(AMASK, AVALUE)  {  enqueue(0,0)  while (queue !=empty)  (AMASK,AVALUE) =0 dequeue( )   collision=IdentifyCmnd(AMASK, AVALUE)  if (collision) then   {    TEMP = AMASK+1    NEW_AMASK = (AMASK>>1)+1   enqueue(NEW_AMASK, AVALUE)    enqueue(NEW_AMASK, AVALUE+TEMP)    }/*endif */  endwhile  }/* return */

The symbol “!=” means not equal to. AVALUE and AMASK would have valuessuch as those indicated in the following table for such code.

AVALUE AMASK 0000 0000 0000 0001 0001 0001 0000 0011 0010 0011 0001 00110011 0011 0000 0111 0100 0111 . . . . . .

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

FIG. 5 illustrates an embodiment wherein the interrogator 26 starts thetree search at a selectable level of the search tree. The search treehas a plurality of nodes 51, 52, 53, 54 etc. at respective levels. Thesize of subgroups of random values decrease in size by half with eachnode descended. The upper bound of the number of devices 12 in the field(the maximum possible number of devices that could communicate with theinterrogator) is determined, and the tree search method is started at alevel 32, 34, 36, 38, or 40 in the tree depending on the determinedupper bound. In one embodiment, the maximum number of devices 12potentially capable of responding to the interrogator is determinedmanually and input into the interrogator 26 via an input device such asa keyboard, graphical user interface, mouse, or other interface. Thelevel of the search tree on which to start the tree search is selectedbased on the determined maximum possible number of wirelessidentification devices that could communicate with the interrogator.

The tree search is started at a level determined by taking the base twologarithm of the determined maximum possible number. More particularly,the tree search is started at a level determined by taking the base twologarithm of the power of two nearest the determined maximum possiblenumber of devices 12. The level of the tree containing all subgroups ofrandom values is considered level zero (see FIG. 5), and lower levelsare numbered 1, 2, 3, 4, etc. consecutively.

By determining the upper bound of the number of devices 12 in the field,and starting the tree search at an appropriate level, the number ofcollisions is reduced, the battery life of the devices 12 is increased,and arbitration time is reduced.

For example, for the search tree shown in FIG. 5, if it is known thatthere are seven devices 12 in the field, starting at node 51 (level 0 )results in a collision. Starting at level 1 (nodes 52 and 53 ) alsoresults in a collision. The same is true for nodes 54, 55, 56, and 57 inlevel 2. If there are seven devices 12 in the field, the nearest powerof two to seven is the level at which the tree search should be started.Log₂ 8=3, so the tree search should be started at level 3 if there areseven devices 12 in the field.

AVALUE and AMASK would have values such as the following assumingcollisions take place from level 3 all the way down to the bottom of thetree.

AVALUE AMASK 0000 0111  0000 1111* 1000 1111* 0100 0111  0100 1111* 11001111*

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

In operation, the interrogator transmits a command requesting devices 12having random values RV within a specified group of random values torespond, the specified group being chosen in response to the determinedmaximum number. Devices 12 receiving the command respectively determineif their chosen random values fall within the specified group and, ifso, send a reply to the interrogator. The interrogator determines if acollision occurred between devices that sent a reply and, if so, createsa new, smaller, specified group, descending in the tree, as describedabove in connection with FIG. 4.

Another arbitration method that can be employed is referred to as the“Aloha” method. In the Aloha method, every time a device 12 is involvedin a collision, it waits a random period of time before retransmitting.This method can be improved by dividing time into equally sized slotsand forcing transmissions to be aligned with one of these slots. This isreferred to as “slotted Aloha.” In operation, the interrogator asks alldevices 12 in the field to transmit their identification numbers in thenext time slot. If the response is garbled, the interrogator informs thedevices 12 that a collision has occurred, and the slotted Aloha schemeis put into action. This means that each device 12 in the field respondswithin an arbitrary slot determined by a randomly selected value. Inother words, in each successive time slot, the devices 12 decide totransmit their identification number with a certain probability.

The Aloha method is based on a system operated by the University ofHawaii. In 1971, the University of Hawaii began operation of a systemnamed Aloha. A communication satellite was used to interconnect severaluniversity computers by use of a random access protocol. The systemoperates as follows. Users or devices transmit at any time they desire.After transmitting, a user listens for an acknowledgment from thereceiver or interrogator. Transmissions from different users willsometimes overlap in time (collide), causing reception errors in thedata in each of the contending messages. The errors are detected by thereceiver, and the receiver sends a negative acknowledgment to the users.When a negative acknowledgment is received, the messages areretransmitted by the colliding users after a random delay. If thecolliding users attempted to retransmit without the random delay, theywould collide again. If the user does not receive either anacknowledgment or a negative acknowledgment within a certain amount oftime, the user “times out” and retransmits the message.

There is a scheme known as slotted Aloha which improves the Aloha schemeby requiring a small amount of coordination among stations. In theslotted Aloha scheme, a sequence of coordination pulses is broadcast toall stations (devices). As is the case with the pure Aloha scheme,packet lengths are constant. Messages are required to be sent in a slottime between synchronization pulses, and can be started only at thebeginning of a time slot. This reduces the rate of collisions becauseonly messages transmitted in the same slot can interfere with oneanother. The retransmission mode of the pure Aloha scheme is modifiedfor slotted Aloha such that if a negative acknowledgment occurs, thedevice retransmits after a random delay of an integer number of slottimes.

Aloha methods are described in a commonly assigned patent applicationnaming Clifton W. Wood, Jr. as an inventor, U.S. patent application Ser.No. 09/026,248, filed Feb. 19, 1998, titled “Method of AddressingMessages and Communications System,” filed concurrently herewith, andnow U.S. Pat. No. 6,275,476, which is incorporated herein by reference.

In one alternative embodiment, an Aloha method (such as the methoddescribed in the commonly assigned patent application mentioned above)is combined with determining the upper bound on a set of devices andstarting at a level in the tree depending on the determined upper bound,such as by combining an Aloha method with the method shown and describedin connection with FIG. 5. For example, in one embodiment, devices 12sending a reply to the interrogator 26 do so within a randomly selectedtime slot of a number of slots.

In another embodiment, levels of the search tree are skipped. Skippinglevels in the tree, after a collision caused by multiple devices 12responding, reduces the number of subsequent collisions without addingsignificantly to the number of no replies. In real-time systems, it isdesirable to have quick arbitration sessions on a set of devices 12whose unique identification numbers are unknown. Level skipping reducesthe number of collisions, both reducing arbitration time and conservingbattery life on a set of devices 12. In one embodiment, every otherlevel is skipped. In alternative embodiments, more than one level isskipped each time.

The trade off that must be considered in determining how many (if any)levels to skip with each decent down the tree is as follows. Skippinglevels reduces the number of collisions, thus saving battery power inthe devices 12. Skipping deeper (skipping more than one level) furtherreduces the number of collisions. The more levels that are skipped, thegreater the reduction in collisions. However, skipping levels results inlonger search times because the number of queries (Identify commands)increases. The more levels that are skipped, the longer the searchtimes. Skipping just one level has an almost negligible effect on searchtime, but drastically reduces the number of collisions. If more than onelevel is skipped, search time increases substantially. Skipping everyother level drastically reduces the number of collisions and savesbattery power without significantly increasing the number of queries.

Level skipping methods are described in a commonly assigned patentapplication 09/026,045 naming Clifton W. Wood, Jr. and Don Hush asinventors, titled “Method of Addressing Messages, Method of EstablishingWireless Communications, and Communications Systems,” filed concurrentlyherewith, now U.S. Pat. No. 6,072,801, and incorporated herein byreference.

In one alternative embodiment, a level skipping method is combined withdetermining the upper bound on a set of devices and starting at a levelin the tree depending on the determined upper bound, such as bycombining a level skipping method with the method shown and described inconnection with FIG. 5.

In yet another alternative embodiment, both a level skipping method andan Aloha method (as described in the commonly assigned applicationsdescribed above) are combined with the method shown and described inconnection with FIG. 5.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of establishing wireless communications between aninterrogator and individual ones of multiple wireless identificationdevices, the wireless identification devices having respectiveidentification numbers and being addressable by specifyingidentification numbers with any one of multiple possible degrees ofprecision, the method comprising utilizing a tree search in anarbitration scheme to determine a degree of precision necessary toestablish one-on-one communications between the interrogator andindividual ones of the multiple wireless identification devices, asearch tree being defined for the tree search method, the tree havingmultiple selectable levels respectively representing subgroups of themultiple wireless identification devices, the level at which a treesearch starts being variable the method further comprising starting thetree search at any selectable level of the search tree.
 2. A method inaccordance with claim 1 and further comprising determining the maximumpossible number of wireless identification devices that couldcommunicate with the interrogator, and selecting a level of the searchtree based on the determined maximum possible number of wirelessidentification devices that could communicate with the interrogator. 3.A method in accordance with claim 2 and further comprising starting thetree search at a level determined by taking the base two logarithm ofthe determined maximum possible number, wherein the level of the treecontaining all subgroups is considered level zero, and lower levels arenumbered consecutively.
 4. A method in accordance with claim 2 andfurther comprising starting the tree search at a level determined bytaking the base two logarithm of the determined maximum possible number,wherein the level of the tree containing all subgroups is consideredlevel zero, and lower levels are numbered consecutively, and wherein themaximum number of devices in a subgroup in one level is half of themaximum number of devices in the next higher level.
 5. A method inaccordance with claim 2 and further comprising starting the tree searchat a level determined by taking the base two logarithm of the power oftwo nearest the determined maximum possible number, wherein the level ofthe tree containing all subgroups is considered level zero, and lowerlevels are numbered consecutively, and wherein the maximum number ofdevices in a subgroup in one level is half of the maximum number ofdevices in the next higher level.
 6. A method in accordance with claim 1wherein the wireless identification device comprises an integratedcircuit including a receiver, a modulator, and a microprocessor incommunication with the receiver and modulator.
 7. A method of addressingmessages from an interrogator to a selected one or more of a number ofcommunications devices, the method comprising: establishing forrespective devices unique identification numbers respectively having afirst predetermined number of bits; establishing a second predeterminednumber of bits to be used for random values; causing the devices toselect random values, wherein respective devices choose random valuesindependently of random values selected by the other devices;determining the maximum number of devices potentially capable ofresponding to the interrogator; transmitting a command from theinterrogator requesting devices having random values within a specifiedgroup of random values to respond, by using a subset of the secondpredetermined number of bits, the specified group being chosen inresponse to the determined maximum number; receiving the command atmultiple devices, devices receiving the command respectively determiningif the random value chosen by the device falls within the specifiedgroup and, if so, sending a reply to the interrogator; and determiningusing the interrogator if a collision occurred between devices that senta reply and, if so, creating a new, smaller, specified group.
 8. Amethod of addressing messages from an interrogator to a selected one ormore of a number of communications devices in accordance with claim 7wherein sending a reply to the interrogator comprises transmitting theunique identification number of the device sending the reply.
 9. Amethod of addressing messages from an interrogator to a selected one ormore of a number of communications devices in accordance with claim 7wherein sending a reply to the interrogator comprises transmitting therandom value of the device sending the reply.
 10. A method of addressingmessages from an interrogator to a selected one or more of a number ofcommunications devices in accordance with claim 7 wherein sending areply to the interrogator comprises transmitting both the random valueof the device sending the reply and the unique identification number ofthe device sending the reply.
 11. A method of addressing messages froman interrogator to a selected one or more of a number of communicationsdevices in accordance with claim 7 wherein, after receiving a replywithout collision from a device, the interrogator sends a commandindividually addressed to that device.
 12. A method of addressingmessages from an interrogator to a selected one or more of a number ofcommunications devices, the method comprising: causing the devices toselect random values for use as arbitration numbers, wherein respectivedevices choose random values independently of random values selected bythe other devices, the devices being addressable by specifyingarbitration numbers with any one of multiple possible degrees ofprecision; transmitting a command from the interrogator requestingdevices having random values within a specified group of a plurality ofpossible groups of random values to respond, the specified group beingless than the entire set of random values, the plurality of possiblegroups being organized in a binary tree defined by a plurality of nodesat respective levels, wherein the size of groups of random valuesdecrease in size by half with each node descended, wherein the specifiedgroup is below a node on the tree selected based on the maximum numberof devices capable of communicating with the interrogator; receiving thecommand at multiple devices, devices receiving the command respectivelydetermining if the random value chosen by the device falls within thespecified group and, if so, sending a reply to the interrogator; and, ifnot, not sending a reply; and determining using the interrogator if acollision occurred between devices that sent a reply and, if so,creating a new, smaller, specified group by descending in the tree. 13.A method of addressing messages from an interrogator to a selected oneor more of a number of communications devices in accordance with claim12 and further including establishing a predetermined number of bits tobe used for the random values.
 14. A method of addressing messages froman interrogator to a selected one or more of a number of communicationsdevices in accordance with claim 13 wherein the predetermined number ofbits to be used for the random values comprises an integer multiple ofeight.
 15. A method of addressing messages from an interrogator to aselected one or more of a number of communications devices in accordancewith claim 13 wherein devices sending a reply to the interrogator do sowithin a randomly selected time slot of a number of slots.
 16. A methodof addressing messages from an interrogator to a selected one or more ofa number of RFID devices, the method comprising: establishing forrespective devices a predetermined number of bits to be used for randomvalues, the predetermined number being a multiple of sixteen; causingthe devices to select random values, wherein respective devices chooserandom values independently of random values selected by the otherdevices; transmitting a command from the interrogator requesting deviceshaving random values within a specified group of a plurality of possiblegroups of random values to respond, the specified group being equal toor less than the entire set of random values, the plurality of possiblegroups being organized in a binary tree defined by a plurality of nodesat respective levels, wherein the maximum size of groups of randomvalues decrease in size by half with each node descended, wherein thespecified group is below a node on a level of the tree selected based onthe maximum number of devices known to be capable of communicating withthe interrogator; receiving the command at multiple devices, devicesreceiving the command respectively determining if the random valuechosen by the device falls within the specified group and, only if so,sending a reply to the interrogator, wherein sending a reply to theinterrogator comprises transmitting both the random value of the devicesending the reply and the unique identification number of the devicesending the reply; using the interrogator to determine if a collisionoccurred between devices that sent a reply and, if so, creating a new,smaller, specified group using a level of the tree different from thelevel used in the interrogator transmitting, the interrogatortransmitting a command requesting devices having random values withinthe new specified group of random values to respond; and if a replywithout collision is received from a device, the interrogatorsubsequently sending a command individually addressed to that device.17. A method of addressing messages from an interrogator to a selectedone or more of a number of RFID devices in accordance with claim 16 andfurther comprising determining the maximum possible number of wirelessidentification devices that could communicate with the interrogator. 18.A method of addressing messages from an interrogator to a selected oneor more of a number of RFID devices in accordance with claim 16 whereinselecting the level of the tree comprises taking the base two logarithmof the determined maximum possible number, wherein a level of the treecontaining all subgroups is considered level zero, and lower levels arenumbered consecutively.
 19. A method of addressing messages from aninterrogator to a selected one or more of a number of RFID devices inaccordance with claim 16 wherein selecting the level of the treecomprises taking the base two logarithm of the determined maximumpossible number, wherein a level of the tree containing all subgroups isconsidered level zero, and lower levels are numbered consecutively, andwherein the maximum number of devices in a subgroup in one level is halfof the maximum number of devices in the next higher level.
 20. A methodof addressing messages from an interrogator to a selected one or more ofa number of RFID devices in accordance with claim 16 wherein selectingthe level of the tree comprises taking the base two logarithm of thepower of two nearest the determined maximum possible number, wherein thelevel of the tree containing all subgroups is considered level zero, andlower levels are numbered consecutively, and wherein the maximum numberof devices in a subgroup in one level is half of the maximum number ofdevices in the next higher level.
 21. A method of addressing messagesfrom an interrogator to a selected one or more of a number of RFIDdevices in accordance with claim 16 wherein the wireless identificationdevice comprises an integrated circuit including a receiver, amodulator, and a microprocessor in communication with the receiver andmodulator.
 22. A method of addressing messages from an interrogator to aselected one or more of a number of RFID devices in accordance withclaim 16 and further comprising, after the interrogator transmits acommand requesting devices having random values within the new specifiedgroup of random values to respond, determining, using devices receivingthe command, if their chosen random values fall within the new smallerspecified group and, if so, sending a reply to the interrogator.
 23. Amethod of addressing messages from an interrogator to a selected one ormore of a number of RFID devices in accordance with claim 22 and furthercomprising, after the interrogator transmits a command requestingdevices having random values within the new specified group of randomvalues to respond, determining if a collision occurred between devicesthat sent a reply and, if so, creating a new specified group andrepeating the transmitting of the command requesting devices havingrandom values within a specified group of random values to respond usingdifferent specified groups until all of the devices withincommunications range are identified.
 24. A communications systemcomprising an interrogator, and a plurality of wireless identificationdevices configured to communicate with the interrogator in a wirelessfashion, the wireless identification devices having respectiveidentification numbers, the interrogator being configured to employ atree search in a search tree having multiple selectable levels, todetermine the identification numbers of the different wirelessidentification devices with sufficient precision so as to be able toestablish one-on-one communications between the interrogator andindividual ones of the multiple wireless identification devices, whereinthe interrogator is configured to start the tree search at anyselectable level of the search tree.
 25. A communications system inaccordance with claim 24 wherein the tree search is a binary treesearch.
 26. A communications system in accordance with claim 24 whereinthe wireless identification device comprises an integrated circuitincluding a receiver, a modulator, and a microprocessor in communicationwith the receiver and modulator.
 27. A system comprising: aninterrogator; a number of communications devices capable of wirelesscommunications with the interrogator; means for establishing apredetermined number of bits to be used as random numbers, and forcausing respective devices to select random numbers respectively havingthe predetermined number of bits; means for inputting a predeterminednumber indicative of the maximum number of devices possibly capable ofcommunicating with the receiver; means for causing the interrogator totransmit a command requesting devices having random values within aspecified group of random values to respond, the specified group beingchosen in response to the inputted predetermined number; means forcausing devices receiving the command to determine if their chosenrandom values fall within the specified group and, if so, send a replyto the interrogator; and means for causing the interrogator to determineif a collision occurred between devices that sent a reply and, if so,create a new, smaller, specified group.
 28. A system in accordance withclaim 27 wherein sending a reply to the interrogator comprisestransmitting the random value of the device sending the reply.
 29. Asystem in accordance with claim 27 wherein the interrogator furtherincludes means for, after receiving a reply without collision from adevice, sending a command individually addressed to that device.
 30. Asystem comprising: an interrogator configured to communicate to aselected one or more of a number of communications devices; a pluralityof communications devices; the devices being configured to select randomvalues, wherein respective devices choose random values independently ofrandom values selected by the other devices, different sized groups ofdevices being addressable by specifying random values with differinglevels of precision; the interrogator being configured to transmit acommand requesting devices having random values within a specified groupof a plurality of possible groups of random values to respond, thespecified group being less than the entire set of random values, theplurality of possible groups being organized in a binary tree defined bya plurality of nodes at respective levels, wherein the size of groups ofrandom values decrease in size by half with each node descended, whereinthe specified group is below a node on the tree selected based on apredetermined maximum number of devices capable of communicating withthe interrogator; devices receiving the command being configured torespectively determine if their chosen random values fall within thespecified group and, if so, send a reply to the interrogator; and, ifnot, not send a reply; and the interrogator being configured todetermine if a collision occurred between devices that sent a reply and,if so, create a new, smaller, specified group by descending in the tree.31. A system in accordance with claim 30 wherein the random valuesrespectively have a predetermined number of bits.
 32. A system inaccordance with claim 30 wherein respective devices are configured tostore unique identification numbers of a predetermined number of bits.33. A system in accordance with claim 30 wherein respective devices areconfigured to store unique identification numbers of sixteen bits.
 34. Asystem comprising: an interrogator configured to communicate to aselected one or more of a number of RFID devices; a plurality of RFIDdevices, respective devices being configured to store uniqueidentification numbers respectively having a first predetermined numberof bits, respective devices being further configured to store a secondpredetermined number of bits to be used for random values, respectivedevices being configured to select random values independently of randomvalues selected by the other devices; the interrogator being configuredto transmit an identify command requesting a response from deviceshaving random values within a specified group of a plurality of possiblegroups or random values, the specified group being less than or equal tothe entire set of random values, the plurality of possible groups beingorganized in a binary tree defined by a plurality of nodes at respectivelevels, wherein the maximum size of groups of random values decrease insize by half with each node descended, wherein the specified group isbelow a node on a level of the tree selected based on the maximum numberof devices known to be capable of communicating with the interrogator;devices receiving the command respectively being configured to determineif their chosen random values fall within the specified group and, onlyif so, send a reply to the interrogator, wherein sending a reply to theinterrogator comprises transmitting both the random value of the devicesending the reply and the unique identification number of the devicesending the reply; the interrogator being configured to determine if acollision occurred between devices that sent a reply and, if so, createa new, smaller, specified group using a level of the tree different fromthe level used in previously transmitting an identify command, theinterrogator transmitting an identify command requesting devices havingrandom values within the new specified group of random values torespond; and the interrogator being configured to send a commandindividually addressed to a device after communicating with a devicewithout a collision.
 35. A system in accordance with claim 34 whereinthe interrogator is configured to input and store the predeterminednumber.
 36. A system in accordance with claim 34 wherein the devices areconfigured to respectively determine if their chosen random values fallwithin a specified group and, if so, send a reply, upon receivingrespective identify commands.
 37. A system in accordance with claim 36wherein the interrogator is configured to determine if a collisionoccurred between devices that sent a reply in response to respectiveidentify commands and, if so, create further new specified groups andrepeat the transmitting of the identify command requesting deviceshaving random values within a specified group of random values torespond using different specified groups until all responding devicesare identified.
 38. A method, comprising: transmitting, from a reader,an initial wireless command to start identification of a plurality ofradio frequency identification (RFID) tags, the initial wireless commandspecifying at least two bits and requesting first RFID tags having theat least two bits to reply with at least random numbers generated on thefirst RFID tags as identifiers to be used by the reader in subsequentcommunications to individually address the first RFID tags; determiningwhether there is a collision in response to the initial wirelesscommand; identifying, from a response to the initial command, a randomnumber generated at an RFID tag, if there is no collision in response tothe initial wireless command; and transmitting, from the reader, asubsequent wireless command to identify RFID tags, the subsequentcommand specifying at least the two bits to request replies.
 39. Themethod of claim 38, wherein the first RFID tags are to select timeslots, based on random numbers generated on the first RFID tags, toreply to the initial wireless command.
 40. The method of claim 39,further comprising: transmitting, from the reader, at least one commandto indicate the time slots to the first RFID tags.
 41. The method ofclaim 38, wherein the random number is a sixteen-bit random number. 42.The method of claim 38, wherein the subsequent wireless command includesone bit more than the at least two bits specified in the initialwireless command.
 43. The method of claim 38, further comprising:transmitting, from the reader, an acknowledge command in response to therandom number being identified from the response.
 44. The method ofclaim 38, wherein the RFID tag is to further communicate to the readerat least a portion of an identification code of the RFID tag.
 45. Aradio frequency identification (RFID) interrogator, comprising: one ormore antennas; a controller; a transmitter coupled to the controller andthe one or more antennas to send a first wireless radio frequency (RF)signal to start identification of individual tags of a population ofRFID tags, the first RF signal requesting RFID tags having first bitsspecified in the first RF signal to reply, the first bits having atleast two bits; and a receiver coupled to the controller and the one ormore antennas to detect a collision in response to the first RF signaland, when there is no collision in response to the first RF signal, todetermine an identifier of a first RFID tag from a reply to the first RFsignal; wherein the transmitter is to subsequently use the identifier,determined from the reply to the first RF signal, to address the firstRFID tag, among the population of RFID tags, for a response from thefirst RFID tag.
 46. The interrogator of claim 45, wherein thetransmitter is to further send a second wireless RF signal to specify atleast the first bits and to request RFID tags having bits specified inthe second RF signal to reply.
 47. The interrogator of claim 45, whereinthe first RF signal requests the RFID tags having the first bitsspecified in the first RF signal to reply with at least random numbersgenerated on respective RFID tags; and the first RFID tag is identifiedvia a random number provided by the first RFID tag in the reply to thefirst RF signal.
 48. The interrogator of claim 45, wherein a randomnumber provided by the first RFID tag in the reply to the first RFsignal has sixteen bits.
 49. The interrogator of claim 45, wherein thetransmitter is to further send an acknowledge signal to the first RFIDtag, in response to a random number being identified from the reply. 50.A radio frequency identification (RFID) system, comprising: a pluralityof RFID tags; and an interrogator having a range for wirelesscommunications, the plurality of RFID tags disposed within the range forcommunications with the interrogator, the interrogator comprising: atleast one antenna, a transmitter coupled to the at least one antenna totransmit a first wireless radio frequency (RF) signal to initiate asearch to identify the RFID tags, the first RF signal specifying atleast two bits, wherein RFID tags having the at least two bits reply tothe first RF signal with at least random numbers generated on respectiveRFID tags, and a receiver coupled to the at least one antenna toidentify, from at least one reply to the first RF signal, a randomnumber generated by a first RFID tag, if there is no response collisionin replying to the first RF signal; wherein the transmitter is tosubsequently use the random number, identified from the reply to thefirst RF signal, to request a response from the first RFID tag.
 51. TheRFID system of claim 50, wherein each of the RFID tags having the atleast two bits generates a random value to determine a time slot toreply.
 52. The RFID system of claim 51, wherein the transmitter is tofurther transmit a plurality of second signals to indicate a pluralityof time slots to reply.
 53. The RFID system of claim 50, wherein thetransmitter is to transmit a second signal to cause the first RFID tagto generate the random number as an identifier.
 54. The RFID system ofclaim 53, wherein the second signal is different from the first signal.55. The RFID system of claim 50, wherein the random number is asixteen-bit number.
 56. The RFID system of claim 50, wherein the atleast two bits are a portion of the random number.
 57. A radio frequencyidentification (RFID) system, comprising: an interrogator to transmit aninitial wireless radio frequency (RF) signal to start a search toidentify RFID tags, the initial wireless RF signal specifying at leasttwo first bits and requesting replies; and a set of RFID tags, each tagof the set having: an antenna, a memory storing a plurality of bits; anda circuit coupled to the antenna to receive the initial RF signal, tocompare the at least two first bits with corresponding bits stored inthe memory, to independently generate a random number as an identifier,to generate a random value to select a time slot to reply, and to replywith the random number in accordance with the time slot, if there is amatch between the at least two first bits specified in the initial RFsignal and the corresponding bits stored in the memory; wherein theinterrogator is to individually address a first RFID tag among the setof RFID tags, using the random number of the first RFID tag identifiedfrom a reply to the first wireless RF signal, to request a response fromthe first RFID tag.
 58. The system of claim 57, wherein the interrogatoris to further transmit at least one signal to indicate subsequent timeslots for RFID tags having the at least two first bits to reply.
 59. Thesystem of claim 57, wherein the interrogator is to transmit a separatesignal to cause each tag of the set to generate the random number. 60.The system of claim 59, wherein the random number is a sixteen-bitsnumber.
 61. The system of claim 59, wherein the interrogator is tofurther transmit an acknowledge signal if a first RFID tag is identifiedfrom a response to the initial RF signal.
 62. The system of claim 57,wherein each tag of the set is to transmit the random number viabackscattering.
 63. A radio frequency identification (RFID) method,comprising: transmitting, from a reader, a first wireless command toinitiate identification of a population of RFID tags and a plurality ofsubsequent wireless commands to continue the identification of apopulation of RFID tags, the first command including first bits, thefirst command to request a set of RFID tags having the first bits toreply with identifiers of the set of RFID tags, the identifiersincluding random numbers individually generated by the set of RFID tags,the first bits including at least two bits; generating, by the set ofRFID tags, the random numbers independent from each other; generating,by the set of RFID tags, random values; replying, by the set of RFIDtags, to the first command and the subsequent command with at least therandom numbers of the set of RFID tags, in an order in accordance withthe random values; receiving, at the reader, a reply to the firstcommand from a first RFID tag; determining whether there is a collisionin replying to the first command; if there is no collision in replyingto the first command, identifying from the reply a random numbergenerated by the first RFID tag; and transmitting a second wirelesscommand to address the first RFID tag using the random number, thesecond wireless command to request a response from the first RFID tagaddressed by the random number.
 64. The method of claim 63, furthercomprising: transmitting a third wireless command from the reader tocontinue identification of a population of RFID tags, the third commandincluding at least the first bits included in the first command.
 65. Themethod of claim 64, wherein the third command includes one more bit thanthe first bits to address RFID tags.
 66. The method of claim 63, whereinthe random number is a sixteen-bit random number.
 67. The method ofclaim 63, wherein the first bits is a portion of the random number. 68.The method of claim 63, wherein the subsequent commands comprisecoordination pulses to indicate time slots.
 69. The method of claim 63,wherein the first RFID tag further transmits at least a portion of anidentification code to the reader.
 70. The method of claim 63, whereineach of the subsequent wireless commands continues the request of thefirst wireless command.
 71. The method of claim 63, wherein each of thesubsequent wireless commands indicates a time slot for replying inaccordance with the request of the first wireless command.
 72. Themethod of claim 63, further comprising: transmitting, from the reader,an acknowledge command in response to the random number being identifiedfrom the reply.
 73. A radio frequency communications-based method ofconducting a financial transaction, comprising: sending a first wirelessradio frequency (RF) signal to start identification of one or more radiofrequency devices of a population of radio frequency devices, the firstRF signal requesting one or more radio frequency devices having at leasttwo first bits specified in the first RF signal to reply; receiving aresponse via a receiver coupled to a controller and one or moreantennas, said receiver, said controller and said one or more antennasconfigured to detect a collision in response to the first RF signal and,when there is no collision in response to the first RF signal, todetermine an identifier of a first radio frequency device from a replyto the first RF signal; addressing the first radio frequency deviceusing the identifier determined from the reply to the first RF signal soas to elicit a subsequent response from the first radio frequencydevice; and initiating a financial transaction based at least in part onsaid acts of sending, receiving and addressing, thereby resulting in thedebiting of an account associated with said first radio frequencydevice.
 74. The method of claim 73, wherein the financial transaction isassociated with the payment of a toll.
 75. The method of claim 74,wherein said receiver and said one or more antennas is disposed within atoll booth, and said method further comprises operating said receiverdisposed within said toll booth at least when said first radio frequencydevice issuing said response to said first wireless RF signal is inproximity thereto.
 76. The method of claim 74, wherein the financialtransaction comprises receiving a credit card number against which thetoll can be charged.
 77. The method of claim 73, wherein the debiting ofthe account comprises charging a credit card number associated with anowner of the account.
 78. The method of claim 73, wherein the financialtransaction is for payment for goods or services.
 79. The method ofclaim 78, further comprising: transmitting a subsequent wireless commandrequesting one or more responses to continue the identification of oneor more radio frequency devices within the population of radio frequencydevices, the subsequent wireless command to identify a subset of thepopulation of radio frequency devices and request the subset to replywith identification numbers.
 80. The method of claim 73, wherein theresponse comprises further information about the first radio frequencycommunications device.
 81. The method of claim 73, wherein theidentifier comprises a unique identification code that uniquelyidentifies the first radio frequency device among the population ofradio frequency devices.
 82. The method of claim 73, wherein theidentifier comprises a random number generated by the first radiofrequency device.
 83. The method of claim 73, wherein the first radiofrequency device is configured to select a random value that determinesa time slot in which the first radio frequency device provides theresponse.